Self-foraging vs facilitated foraging by Lesser Black-backed Gull (Larus fuscus) at the Frisian Front, the Netherlands

The Lesser Black-backed Gull, Larus fuscus Linnaeus, 1758, is a surface feeder with a broad prey spectrum that forages in the North Sea on subsurface pelagic fishes and crabs swimming within 0.6 m of the sea surface. In July 2006 and in July/August 2009, we surveyed the Frisian Front area in the southern North Sea (53.4°N–54.3°N, 4.0°E–5.5°E) to determine the prey base for Lesser Black-backed Gulls. In July/August 2009, we conducted bird counts in an integrated ecosystem survey by simultaneously sampling subsurface pelagic prey by a combination of fishing and echosounding. Lesser Black-backed Gulls make use of facilitated foraging to improve the catchability of their prey. The two most important aids for facilitated foraging by Lesser Black-backed Gull in the Frisian Front were (1) multispecies feeding associations with Common Murre, Uria aalge (Pontoppidan, 1763), and (2) fisheries discarding bycatch and offal. We found that in the top meter of the water column, the natural prey base is large enough to provide for self-foraging; however, Lesser Black-backed Gulls were found to rely mainly on facilitated foraging.